1. Field of the Invention
The present invention relates generally to siloxane compositions that exhibit stability to moisture and oxygen. More particularly, the present invention is directed to organohydrosiloxanes stabilized with one or more antioxidant compounds.
2. Description of the Prior Art
1,3,5,7-Tetramethylcyclotetrasiloxane (TMCTS) is one example of an organohydrosiloxane (i.e., constructed of [—HSiR—O—] units where R is alkyl, and in this case, methyl) that can be used for specialty polymer formation. TMCTS is an example of a cyclic organohydrosiloxane, and it has the ability to undergo ring opening polymerization. It also has reactive Si—H bonds that can be used for adding functionality to the resulting polymer. Due to these unique properties, the semiconductor industry recently chose TMCTS as a new precursor for the plasma enhanced chemical vapor deposition (PECVD) of Low K dielectric films on semiconductor wafers by chemical vapor deposition.
The introduction of TMCTS and similar cyclic organohydrosiloxanes into the semiconductor fab has been causing serious manufacturing challenges because these materials can polymerize unpredictably. This behavior can be very costly and time consuming if polymerization occurs inside liquid or vapor chemical delivery lines where the only remedy is to disassemble and replace the affected lines. The semiconductor industry demands a stable, predictable and reliable product, and this behavior is unacceptable for high volume semiconductor manufacturing. Therefore, it is necessary to find a means to stabilize TMCTS to ensure that the product does not polymerize during transport from the chemical supplier to the end-use process, even after exposure to various conditions.
TMCTS and other alkyl-hydrogen linear or cyclic siloxanes having Si—H bonds can react with oxygen or other components of air. This reaction can cause significant degradation of the siloxane, and when cyclic materials are involved, complete polymerization can eventually result, even after brief air exposure at room temperature.
Degradation of the chemical reduces shelf life, lowers product quality, and can even cause greater problems such as equipment downtime if the material polymerizes in chemical processing equipment. Safety issues are also a concern if pressurized chemical lines and valves become blocked.
For the semiconductor industry, degradation of an organohydrosiloxane product to higher molecular weight species causes additional problems with chemical use. The most common method to introduce high boiling point (e.g., >50° C.) materials into a processing tool is by directly injecting a vapor or mist of chemical into a heated “carrier” gas stream that flows into a process chamber below ambient pressure. The heated stream of carrier gas vaporizes the product, and the product vapor is transported to the process chamber where the siloxane is transformed into a carbon doped silicon oxide film. However, this technique usually involves a single, pure species in order to have homogeneous product vapor inside the equipment. If high molecular weight species are present, as found in degraded organohydrosiloxanes, they do not readily vaporize in the heated gas stream, but instead simply collect on the internal surfaces of the process equipment. This leads to several problems, including, for example, clogged transfer lines and valves and particle formation on a wafer.
There have been prior art attempts to stabilize siloxane and silane-containing compositions. U.S. Pat. No. 5,177,142 to Stein et al. discloses silicone release coating compositions comprising a polydiorganosiloxane containing unsaturated group, a crosslinking agent containing Si—H groups, and a hydrosilation catalyst. The catalyst is inhibited by incorporating at least one free radical inhibitor, such as alkoxyphenols, in the composition. As a result, the compositions have a reduced tendency to bond to acrylic adhesives.
U.S. Pat. No. 5,556,901 to Bryson discloses a polysilane-based composition used particularly for producing silicon carbide. The composition comprises polysilane chains capable of being degraded by oxidation and the formation of polysiloxane-type products, and at least one antioxidant system. The antioxidant system greatly reduces the sensitivity to air of the polysilanes.
What is lacking in the prior art is a composition, like the present invention, comprising organohydrosiloxanes that exhibits improved stability to moisture and oxygen, for use in the semiconductor field. The stable composition of the present invention significantly extends the shelf life of organohydrosiloxane products and provides greater flexibility in handling these products in chemical processes or semiconductor manufacturing. The resulting stabilization of siloxanes prevents the possibility of complete polymerization (i.e., solidification) of product in chemical delivery lines or valves, which lowers equipment maintenance and costs and reduces time the machinery is out of production. In addition, reducing the formation of higher molecular weight compounds allows homogeneous vaporization of the product without concern for the gradual deposition of higher molecular weight compounds in vapor delivery lines leading to more consistent, higher quality deposited films.